Along with the greatly increased use of computers for offices and for manufacturing facilities, there has developed a need for a cable which may be used to connect peripheral equipment to mainframe computers and to connect two or more computers into a common network. Of course, given the ever-increasing demands for data transmission, the sought-after cable desirably should not only provide substantially error-free transmission at relatively high rates but also satisfy numerous elevated operational performance criteria. Specifically, the particular cable design of the present invention consistently performs at operational levels which exceed the transmission requirements for cables qualifying as Category 5 cables under TIA/EIA-568A. Among the elevated operational performance criteria that the cable of this invention can reliably and consistently exhibit over existing standards criteria are higher crosstalk margins, i.e. over at least about 10 dB for Near End Crosstalk (NEXT) and over at least about 8 dB for Power Sum Crosstalk (PSUM NEXT), as well as improved Structural Return Loss (SRL) margins, i.e. over at least about 3 dB.
Not surprisingly, of importance to the design of metallic-conductor cables for use in local area networks are the speed and the distances over which data signals must be transmitted. In the past, this need had been one for interconnections operating at data speeds up to 20 kilobits per second and over a distance not exceeding about 150 feet. This need was satisfied with single jacket cables which may comprise a plurality of insulated conductors that were connected directly between a computer, for example, and receiving means such as peripheral equipment. Currently, equipment, generally identified throughout the industry as Category 3 products, is commercially available that can effectively transmit up to 16 MHz data signals and a series of products designated as Category 5 provide the capability of effectively transmitting up to 100 MHz data signals. However, further advances in data rate capability are becoming increasingly difficult because of the amount of crosstalk between the conductor pairs of such commercially available single-jacketed, twisted-pair cables.
Additionally, for both operational and costs reasons, it is important whether or not the system is arranged to provide transmission in what is called a balanced mode. In balanced mode transmission, voltages and currents on the conductors of a pair are equal in amplitude but opposite in polarity. To accomplish this balanced mode transmission, additional components, such as transformers, for example, at end points of the cable between the cable and logic devices may be required, thereby increasing the cost of the system. Oftentimes, computer equipment manufacturers have preferred the use of systems characterized by an unbalanced mode to avoid investing in additional components for each line. At the same time, however, peripheral connection arrangements, specifically the cabling used therein, must meet predetermined attenuation and crosstalk requirements.
As an alternative to a single-jacketed, twisted-pair cable, sometimes the cabling needs of the communications industry have been filled with coaxial cable comprising the well-known center solid and outer tubular conductor separated by a dielectric material. However, coaxial cables, not only inherently provide unbalanced transmission, but also present several other problems. Among other concerns, coaxial connectors are relatively expensive and difficult to install and connect, and, unless they are well designed, installed and maintained, can be the cause of electromagnetic interference.
Given their increasingly stringent objectives, customers, local area network (LAN) vendors and distribution system vendors continue to explore alternatives for making LAN wiring more affordable and manageable while still providing the necessary level of transmission performance. Previously overlooked by some investigators has been the unshielded twisted pair long used for premises distribution of telephone signals.
The unshielded twisted pair has long been used for telephone transmission in the balanced (differential) mode. Used in this manner, the unshielded twisted pair has excellent immunity from interference whether from the outside (EMI) or from signals on other pairs (crosstalk). Another point of concern is that the cable be designed so as not to emit electromagnetic radiation from the cable into the surrounding environment. Over the past several years, in fact, some LAN designers, have come to realize the latent transmission capability of unshielded twisted pair wire. Especially noteworthy is the twisted pair's capability to transmit rugged quantized digital signals as compared to corruptible analog signals. The limitations imposed by crosstalk, especially near-end crosstalk, on the data rate/distance capabilities of twisted pair cables are generally recognized.
In an attempt to enhance the operational performance of twisted pair cables, manufacturers have employed a variety of different twist schemes. As used herein, twist scheme is synonymous with what the industry sometimes calls twinning or pairing. In general, twist scheme refers to the exact length and type/lay of twist selected for each conductor pair. More specifically, in one such twist scheme particularly described in commonly-assigned U.S. Pat. No. 4,873,393 issued in the names of Friesen and Nutt and which is hereby expressly incorporated by reference, it is stated that the twist length for each insulated conductor pair should not exceed the product of about forty and the outer diameter of the insulation of one of the conductors of the pair. While this is just one example of an existing approach for defining a twist scheme which results in an enhanced cable design, many others exist. However, the particular twist scheme set forth and claimed herein is believed to be uniquely different from all existing cable designs with specific technical distinctions discussed in greater detail later.
In addition to controlled pair twist schemes, another treatment for crosstalk is to add shielding over each twisted pair to confine its electric and magnetic fields. However, as the electric and magnetic fields are confined, resistance, capacitance and inductance all change, each in such a way as to increase transmission loss. For instance, it is not unusual to find designs of shielded pairs whose attenuation is three times that of similar unshielded pairs.
Seemingly, the solutions of the prior art to the problem of providing a local area network cabling arrangement which can be used to transmit, for example, data bits error-free at relatively high rates over relatively long distances have not yet been totally satisfying for the ever-increasing demands of the communications industry. What is needed and what is not provided by the prior art is a cable which is inexpensively made and which has operational performance levels which significantly surpass the criteria setting forth present standards for high-performance metallic cables, such as TIA/EIA 568A. In particular, the sought-after cable should exhibit substantially higher crosstalk margins and Structural Return Loss (SRL) margins to handle the ever-increasing transmission rates, i.e. 1.24 gigabits per second. In fact, it is believed that the cable design of the present invention is capable of being used in a Gigabit Ethernet system without the need for special electronics.